Flexible Plastic Pushrod Warping - RCCanada - Canada Radio Controlled Hobby Forum
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Old 11-17-2010, 08:59 AM   #1
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Flexible Plastic Pushrod Warping

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Hi all.... just wondering if any of you guys have had this problem.

This summer my brother came down to fly at my airstrip and two of his planes, SSE and Sig 4*, both had pushrod binding problems.

After we investigated a bit, we found that the plastic pushrods that he had used when building had warped drastically and were binding severely and the throws were way out. I am not sure they will ever go back as they were snaked quite badly in the fuse. Must have gotten too warm somewhere along the line. They would be a real pain to change out as he built them into the fuse while building and are not easily accessible. He is going to leave the planes sit over the winter and see if the pushrods "relax", if not he will have to remove the covering and swap them out... more "life tuition" to pay here!

I have had planes go slightly out of trim over the years but I have never seen anything like this. I prefer to use solid wire, hardwood or graphite pushrods in my setups, but I do have a few Arfs that have the plastic setups also.

Something else to check over closely before flying!

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Old 11-17-2010, 09:29 AM   #2
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Re: Flexible Plastic Pushrod Warping

First time I used them in a kit, I was soo glad the model had an airfoil that produced a lot of lift. I had them in my Slo-poke and went into a loop with those and on the dive part of it, the airspeed flattened out the elevator!! I swear it was the spirit of the guy who built it, as it was almost complete when I got it, had a "hand" in making sure it didn't bite the dirt! The maiden was almost the last. Lesson learned! I changed them out for solid rod and its still flying today out of someone else's hanger. Should have kept it for night flying, hehehehe. I will only use the outer jacket of those replacing the innards with steel or CF.
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Old 11-17-2010, 09:49 AM   #3
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Re: Flexible Plastic Pushrod Warping

Yep, same experience. I used them on an aircraft ... once ... and you wouldn't believe the trim changes from mid summer to mid winter!

I ripped them suckers out and went back to the tried and true method of binding steel pushrod ends to balsa (carbon rods these days) with good ole thread 'n glue. pull pull cables for the bigger beasts of course.
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Old 11-17-2010, 10:02 AM   #4
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Re: Flexible Plastic Pushrod Warping

I cant stand Nyrod pushrods and only use them if I ABSOLUTELY have to. Otherwise it is threaded rod-and-Dowel or Pull-Pull.

Nyrod had 2 problems I hate.

1) it expands and contracts with changes in temperature. Like Jim says above- your trims will change from day to day depending on the weather.

2)The other problem is the inner rod tends to stiffen on cold days. This makes the controls hard to move ( if at all) if your rods are snaking through the aircraft. All that snaking means the stiffened inner rod holds the set of the curvature and tends to seize-up if you try to move it beyond its at-rest position.
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Old 11-17-2010, 10:08 AM   #5
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Re: Flexible Plastic Pushrod Warping

The bigger the plane the worse the changes due to heat and cold.

If I replace the inner plastic rods I sometimes cut shorter pieces of piano wire and join them with small pieces of nyrod. This allows them to have less friction if the outer plastic guides are curved inside the fuselage.
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Old 11-17-2010, 12:14 PM   #6
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Re: Flexible Plastic Pushrod Warping

I had the same problem in my SIG MK2. Left them in but I carved out the underside of the fuse and anchored the guide tube to every former...
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Old 11-17-2010, 12:27 PM   #7
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Re: Flexible Plastic Pushrod Warping

etotheo is correct about fixing the outer sheath. If too long a run is left usupported it will flex under load.

I have been using Goldenrods for years this way and have lived with the trim changes. However, I try and limit the arc or bends as much as possible and have been able to run max changes of 15-20 degrees, mostly out near the control surfaces.

In fact, I'm removing the bent wire rods in my Harvard II and installing Goldenrods. That way I can guarantee I will have control under extreme flight loads.

p.s. Along with the unsupported outer sheath, one other common mistake I see is the inner rod hanging too far out of the sheath on the way to the control or servo. This is far more flexible than the outer layer and more likely to bend under load.
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Old 11-18-2010, 11:24 AM   #8
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Re: Flexible Plastic Pushrod Warping

I'm with Cougar on this one.

I use Gold n' Rods regularly and haven't had a problem. I haven't flown those airframes over HUGE ranges of temperature though. I actually find they have less drag than metal rods in plastic sleeves that I've seen others use (particularly in ARFs). I've also encountered lots of drag and local buckling with "steel cable in plastic sleeve" types.

The advantage is that they don't have to be "straight shot". The downside is they really do need to be regularly supported over their length and don't like being bent into sharp curves.

For the 2-56 rods, I set my support interval at less than 4" and applied as much compressive load as I could get away with before breaking something else (like a control surface). So far, so good.

Otherwise, wood and CF have a very low coefficient of expansion that makes them naturally resistant to trim changes with temperature.
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Old 11-18-2010, 12:55 PM   #9
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Re: Flexible Plastic Pushrod Warping

People make a big mistake when they instal flexible pushrods such as "golden rods" etc. They must be FIRMLY anchored at each end, and by this I mean in good plywood, not balsa. The rest of the unit must have a slight bow and be left to move unimpeded.

If the tube is secured the whole length within the fuselage, the expansion/contraction with temperature change will be out the ends, changing trim. If the unit is slack and the ends only are rigid, the expansion/contraction will cause the bow to either increase or decrease and because the coefficient is the same for both parts, not affecting the outcome at the control surface.

I ran tests in the freezer/warmth to demonstrate to myself that the above is true. Think about the throttle cable in your car. It follows the same principle, and is not affected by temperature change. Tex.
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